In the analysis of a sample compound using a gas chromatograph a supply of inert carrier gas (mobile phase) is continually passed as a stream through a heated column containing porous sorptive media (stationary phase). In the past, chromatographic systems have incorporated columns formed as hollow capillary tubes having an inner diameter in the range of few hundred microns (.mu.m, 1.times.10.sup.-6 meters). In such systems, a sample of the subject mixture is injected into the mobile phase stream and passed through the capillary column, which is typically positioned within an oven. As the subject mixture passes through the capillary column, it separates into its various components. Separation is due primarily to differences in the volatility characteristics of each sample component with respect to the temperature in the column. Column temperature is primarily regulated by oven temperature. A detector, positioned at the outlet end of the capillary column, detects each of the separated components as they exit the column.
Column efficiency (typically referred to in terms of theoretical plates) of capillary columns is dependent on both column length and diameter. A column having a larger inner diameter must be longer than a column having a smaller inner diameter in order to achieve comparable efficiency. As will be appreciated by those of ordinary skill, greater column length results in longer analysis times, which can effect sensitivity. However, shorter columns with relatively narrow column inner diameters have been limited in relation to the techniques available for sample injection. Consequently, certain prior gas chromatographic analyses have been a compromise between sensitivity and efficiency in relation to the column and injection technique utilized.
Injecting sample directly into column with a diameter less than 530 .mu.m requires a slender syringe needle. For example, a needle with an outer diameter less than 0.240 mm (240 .mu.m) is used for a 250 .mu.m column. As a lower limit, the slender section of the needle has to be long enough to inject a sample into the section of the column inside the gas chromatograph. However, as explained in U.S. Pat. No. 5,032,151--Klein et al., which is incorporated herein by reference, a slender syringe needle can be bent during the piercing of vial caps or while passing through the inlet septum of the column. There are two needle failure modes: buckling and compression. When the needle punches the vial cap, if the piercing force is low, the deflection, y, of the needle is smaller, than a certain value, y.sub.m, the needle will return to its original equilibrium position (i.e., a straight needle) and be stable. As the piercing force increases (for example, with a thicker vial cap), the deflection becomes larger. When the deflection exceeds y.sub.m the needle will move away from its original equilibrium position and fail. In other words, this unstable situation will cause the needle to buckle. The limit or maximum deflection, y.sub.m, depends on the geometry and material of the needle. Compression failure is another mode of needle failure. If the compression stress of the needle exceeds the yield stress of its material, the needle will fail. However, since the syringe needle is slender, buckling failure will occur under a much lower stress than that for compression failure. Accordingly, buckling is the most frequent failure mode for a typical syringe needle.
The critical load, P.sub.cr, of a given needle, below which the needle would not buckle, can theoretically be calculated using Euler's formula: ##EQU1## Where A, L.sub.e and R are the area, effective length and radius of gyration of the needle. The modulus of elasticity of the needle, E, is a material constant. The Euler equation set forth above, however, is derived from a more generalized equation, and presumes that the boundary conditions include both ends of the needle being hinged, i.e., unable to resist a bending moment. On the other hand, if it is assumed that both ends of the needle are fixed, the Euler equation becomes: ##EQU2## indicating that a change in boundary conditions increased the critical load by a factor of four.
Because a needle passing through a septum and into a column represents a dynamic system having end conditions that are neither fixed nor hinged, the true value of P.sub.cr is difficult to determine theoretically, and, accordingly, it is difficult to determine a maximum theoretical deflection value, Y.sub.m. However, those of ordinary skill will realize that there are values of L.sub.e for a given diameter needle that will deflect elastically and return to a stable position. These values are readily determined empirically.
It has been found, however, that for a relatively narrow column, such as the slender needles discussed above, buckling phenomenon reduce the maximum useful length to less than that which is required for successful injection, even if the maximum elastic deflection, y.sub.m, is permitted. Therefore, it would be desirable to increase the length of a slender syringe needle to a length useful for injection that would not catastrophically fail under load.
In the past, several improvements were made to reduce the chance of needle buckling. A tapered needle improves needle strength by reducing its effective length. Finally, the use of thin membrane vial caps and inlet septa with pre-determined through-holes significantly reduce the piercing force. For example, U.S. Pat. No. 5,032,151--Klein et al., incorporated herein by reference, discloses a system for on-column injection using columns having diameters less than 530 .mu.m. This reference explains that the buckling problem described above limits the diameter of chromatographic columns even though narrower columns will provide better results. It is disclosed that columns having diameters less than 530 .mu.m can be effectively utilized by reducing the effective length of the narrowest portion of the injection needle, thereby maximizing resistance to buckling. A preferred embodiment discloses a capillary column having an inner diameter of 320 .mu.m, and a needle that has a distal portion with an outer diameter of 0.2286 mm (228.6 .mu.m).
Despite the advances disclosed in the Klein et al. reference, there remains a long-felt, but as yet unfilled need to provide methods and apparatus whereby syringe needles of relatively narrow cross-section can be utilized in conjunction with slender columns for gas chromatography. Additionally, it would be particularly desirable to provide methods and apparatus whereby needles of even smaller diameter than those disclosed by Klein et al. could be so used. It is therefore an object of the present invention to provide methods and apparatus for substantially eliminating the buckling of slender syringe needles while retaining an overall length sufficient to be used for on-column injection. Additionally, due to the high number of samples processed, it would be desirable to permit relatively inexpensive septa and vial caps to be used, even though they generate a greater resistive force than other types of septa and caps. It is therefore a further object of the present invention to provide such a system whereby conventional vial caps and other septa may be penetrated using a relatively slender needle.